1. Technical Field
The present invention pertains to purification and filtration of molten polymer and, more particularly, to an improved method and apparatus for changing filters in a continuous polymer extrusion process.
2. Discussion of the Prior Art
In extruding articles of small cross-section from thermoplastic polymers such as polyethylene, Nylon, polyester, polystyrene, etc., it is necessary to filter foreign material from the molten polymer. Two common examples of such articles are synthetic textile fibers and thin plastic films of the type used in packaging or for tapes (e.g., computer tape, sound recording tape, etc.). In the production of synthetic textile fibers, which may have a final diameter of as little as ten microns, a particle of foreign matter of five or more microns diameter is quite likely to cause breakage of the fiber during manufacturing. It is desirable, therefore, to filter out any foreign material above a certain size.
In the manufacture of the polymers themselves, a final step is often a pelletizing operation where a filter is used to remove any impurities from the final pellets. One form of impurity is a so-called "gel", a region in the molten polymer which has much higher than average viscosity due to excessive polymer molecular weight or cross-linking of polymer molecules. For some articles, such as very thin film and textile fibers, the gels degrade the product quality and are desirably removed by filtration, either at the time the polymer is manufactured, or in-line, upstream of the extrusion of the final product. In all of these processes, it is desirable to be able to replace dirty filter media (usually woven wire screen) with clean media without interrupting the extrusion process and without introducing air to the polymer stream, since air introduced to the polymer stream would end up as bubbles in the extruded article, rendering it defective. A polymer filter having the capability of changing media without interrupting the extrusion operation is generally called a "continuous polymer filter" or "continuous screen changer".
Numerous types of polymer filters are known in the prior art. A very common type is the so-called screen changer, an early model of which is shown in U.S. Pat. No. 3,007,199 (Curtis). Screen changers typically have a relatively small area of filter medium or screen for a given flow rate of molten polymer. A filter area of one square inch of screen for flow rates of thirty to seventy pounds per hour of polymer is typical of screen changers. For instance, an extruder of 4.5 inches screw diameter at full production can melt 600 to 1100 pounds per hour of polymer and is commonly fitted with a screen changer using screens of 4.5 inches diameter or 15.9 square inches of area. This yields a flow rate of approximately thirty-eight to sixty-nine pounds per hour per square inch of filter area. It would be very unusual but possible to use a screen changer with screens bigger than eight inches diameter on a 4.5 inch extruder. This would yield fifty square inches of area or twelve to twenty-two pounds per hour of polymer flow per square inch of filter area. In any case, screen changers of this type are suitable to remove dust, dirt, metal particles and pigment agglomerates down to a micron size of about forty, more often one hundred microns.
Gel removal, on the other hand, requires filtration by media having a micron rating of twenty or finer, and typically a greater "depth" or thickness of media is used than when merely removing dirt. Gels are amoeba-like in that they can change shape to pass through normal filter screens and then resume a more compact shape downstream of the screen. The combination of finer media and a greater thickness of media tends to cause a very high pressure drop through the filter unless a large area of filter media is used. For this reason, filters for gel removal normally have one square inch of filter area for each 0.20 to 0.70 pounds per hour of polymer flow. A filter used with a 3.5 inch diameter screw extruder having a melt rate of 350 pounds/hour would have a filter media area of about 1300 square inches, or nine square feet. These large area filters are not only useful for molten polymer, but also for solutions of polymer (so-called dopes). Polymer solutions (as used to make spandex or acrylic fibers) are lower in viscosity than polymer melts, so somewhat less filter area is needed to remove the gels that are common in these dopes.
While a great many types of screen changers and other small area polymer filters exist, nearly all large-area gel filters have a similar construction. One such filter is disclosed in U.S. Pat. No. 3,727,767 (Itter et al). One of the simplest of the commercialized large area filters of this type uses four simple globe-type valves to direct polymer to and from either one of two filter housings. The housing contains a single candle type filter element, but it is customary for larger size filters to have a cluster of seven or more candles in a larger housing. A similar filter system is manufactured by Fuji Filter Manufacturing Company and uses two three-way valve plugs for directing polymer flow, the filter housings being heated in an oven enclosure. The Itter et al patent talks about installing each filter housing in a casing which must be heated, but the patent does not disclose how the heating is accomplished.
A very popular brand of large area polymer filter system has been sold by the Fluid Dynamics Company (now Memtec). It is similar to the Fuji filter system with oven heating of the candle housings, but the Fluid Dynamics "CPF" filter uses two sliding spool valves instead of rotating valve plugs. One common size for these filters is to have seven candle elements in each housing, each candle being a perforated tube covered by pleated screen wire in two or more layers. Candle filters, per se, are well known and are disclosed, for example, in the filtration system illustrated and described in U.S. Pat. No. 3,833,121 (Singleton et al). One popular candle size is 1.38" O. D. by 16" long and has 1.2 to 1.4 square feet of area, or about 9 square feet (1300 square inches) for seven candles. Such a filter can be used with a polymer flow rate of two hundred to one thousand pounds per hour, or 0.20 to 0.77 pounds per hour per square inch of area. This corresponds to the output of an extruder with a screw diameter of 2.5 to 4.5 inches. Much larger filters are made by Memtec and by PTI Technologies Inc. (a subsidiary of HR Textron Inc.). PTI offers filters with on-stream areas up to three hundred sixty five square feet for very high polymer flow rates. These very large filters usually have large spool valves connected to the candle housings via heated piping.
In most of the prior art candle type polymer filter systems, one of two filter housings is on-stream. The other is cleaned, installed and heated to be ready to accept the polymer when the filter medium in the on-stream housing becomes too dirty for continued operation. To switch housings, the valves are operated simultaneously (or nearly simultaneously) and polymer is introduced to the clean housing while flow continues through the dirty housing. All of the filters have means to vent air from the clean housing and completely fill that housing with polymer. Once the clean housing is filled, the valves are moved to the position for full flow through the clean housing, the dirty housing being shut off at its inlet and outlet so that it can be removed for cleaning. The entire housing and its dirty candles are cleaned at a remote location and then the clean parts are reassembled, installed and heated to be ready to switch back on-line. Once a week is a typical time to change housings, but it could be more or less depending on the amount of gels and any foreign material in the polymer. Pressure drop across the candles is usually from about two hundred psig on clean candles to fifteen hundred psig when the filters are due for changing.
A primary problem with prior art filters is the high initial cost of the units. Another problem is the high polymer residence time in the filter and non-uniform residence time, potentially causing thermal degradation when thermally sensitive polymers are used. With some prior art filters operator error can cause polymer flow to be completely shut off to both housings, stopping flow from the extruder and blowing the safety rupture disc that must always be installed at the exit of any extruder. This terminates the extrusion process and requires replacement of the expensive rupture disc. Another typical problem is the need to clean the candle housing as well as the candles when dirty candles are removed.